The present invention pertains to numerically controlled oscillators and in particular to radio frequency numerically controlled oscillators.
Because an increasing number of communications systems transmit digitally encoded information, a need has developed for hardware capable of transmitting and receiving such information at radio frequencies. A particularly important piece of equipment in this regard is a RF generator which is stable, digitally programmable for control by a microprocessor and capable of operating at high frequency on the order of 500 mHz.
One approach to providing a numerically controlled oscillator (NCO) involves using a counter which increments with each pulse of a clock until a terminal value is reached, at which point the counter returns to zero and begins to counting up again. Such an NCO provides a gradually upward sloping level of output until the terminal value is reached at which point the output level rapidly returns to a base level so that a saw-toothed output signal is produced. Counters for NCOs generally include a plurality of adders. These adders may be connected in series or in parallel.
In one approach to providing a rapid NCO, a parallel arrangement of adders is employed in order to avoid delays associated with the need for a "carry" signal to ripple through a series of adders as is required for common structures using parallel addition. However, in order to ensure that the carry is proportional from a least to a most significant bit, the use of a large number of logic gates is required resulting in delays that make such NCOs unsuitable for operation at radio-frequencies.